This invention relates to a method and apparatus for frequency division multiplexing.
In several digital modulation techniques, a group of consecutive data bits in an input data stream Di is represented by a symbol. Different combinations of data bits are represented by different symbols. For example, in the case of the group being composed of three bits, there are eight possible combinations and accordingly there are eight different symbols. One common type of digital modulation employing eight symbols is 8-level phase shift keying (8-PSK), in which the eight different symbols induce eight equiangularly spaced values of phase displacement in the bandpass representation.
In the complex baseband representation, each symbol is conventionally represented by a doublet (I, Q). The I and Q components of the symbol are applied to respective ports of a quadrature modulator that also receives a carrier signal and outputs a signal that is modulated in frequency and/or phase and/or amplitude in accordance with the values of the I and Q components.
Orthogonal frequency division multiplexing (OFDM) is a digital modulation technique in which an input data stream is decomposed into several subsidiary streams, each subsidiary stream is represented by a sequence of symbols, and the several sequences of symbols (up to several thousand sequences) are used to modulate respective carriers of constant frequency. The modulated carriers are summed to produce a transmission signal, which is supplied to a transmitter antenna for transmission to a receiver antenna. Receivers equal in number to the carriers and tuned to the carriers respectively receive and detect the sequences of symbols. Each sequence of symbols is then used to recover the corresponding subsidiary data stream, and the subsidiary data streams are combined in order to recreate the original data stream, which may be an HDTV signal. The carriers are sufficiently spaced in frequency that they are orthogonal, i.e. each receiver sees only its own carrier.